Unable to delete keyframes
To unlock or lock a keyframe, you can do three ways.

1) click on the icon symbol to unlock the keyframes for the entire object





2) hover over the keyframes for the selected object press tab once, if grayed out the keys are locked if not then the keys are not locked. 

3) if your mouse cursor is outside the timeline boarders (gray part of the timeline) with this icon greyed then those keyframes are locked.  


and including the dope sheet 



If somehow you are still stuck, just go into the dope sheet then Shift + Right Click select the key(s) you want to delete



Next tap X while hovering over the dope sheet Select left click the "delete keyframes".